This invention relates to electrical conductors, and in particular to a process of forming thin-film metal conductors on a substrate by directly printing thereon metal chelate inks and decomposing the inks.
Thin metal films have a wide variety of applications ranging from interconnects in semiconductor device manufacture, including particle based contacts to photovoltaic semiconductors, to the optical tailoring of glass monoliths and to gas permeable membranes in separations technology. As a result, conventional processes have looked toward optimizing process design and in the synthesis of new inorganic, metal-organic and organometallic compounds specifically for use as thin film precursor materials. Optimization desirably includes providing high purity films of acceptable conductivity while eliminating conventional processing steps in order to reduce costs. It is also desirable to eliminate the photolithography and mask preparation steps used in screen printing and vacuum application, both of which are not conformal. In particular, inks which are amenable to low temperature deposition, such as ink jet printing, screen printing and other direct write approaches, are desirable in order to eliminate the use of costly vacuum application systems. Low temperature deposition is also desirable in the formation of semiconductors, particle-based contacts to photovoltaic semiconductors, and in spray printing on conformal substrates, such as flexible circuit boards, because high-temperature sintering cannot be performed due to thermal limitations associated with the underlying layers. For example, the thermal treatment of a Ni contact onto a ZnO conducting layer, as the top layer in a CuInSe2 (xe2x80x9cCISxe2x80x9d) solar cell, is limited to xcx9c200xc2x0 C. for 2 minutes because of the thermal instability of the underlying solar cell device. It has also been found that when a 1,2-propanediol slurry of Ni powder is deposited onto a conducting ZnO film and annealed in air at 200xc2x0 C. for 2 minutes, the resultant Ni contact becomes crumbly in structure and is not electrically conductive. Moreover, the demand for improved performance in integrated circuits has led to integration of an increasing number of semiconductor devices on chips of decreasing size. This has been achieved by scaling down the device feature size, while increasing the number of interconnect layers. As a result, the topography has become much more severe with each successive device generation. In addition, as metal linewidths shrink, device speed is expected to be limited by the interconnect performance.
Copper is a widely applied electronic material with low bulk resistivity of xcx9c2 xcexcxcexa9xc2x7cm. Many direct write approaches, to the formation of copper conductors, are limited by impurity phase formation. For example, copper (II) carboxylate analogs to Ag(neodecanoate) produce copper oxide when heated to decomposition in air. Most Cu(II) precursor chemistries, however, require relatively high-temperatures (e.g., Cu(hfa)2 yields Cu at 340-400xc2x0 C.) or subsequent processing in the presence of a reducing species (e.g., hydrogen gas) to produce metallic layers. The chemistry of Cu(I) complexes, as chemical-vapor-deposition precursors to Cu films, has also been evaluated for use in the next-generation ultra-large scale integrated circuits. Copper (I) complexes, based on Cu(hfa).L (where hfa-hexafluoroacetylacetonate and L=CO, phosphine, alkene, or alkyne), have been shown, conventionally, to produce copper films by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) at low temperature (100-150xc2x0 C.) with resistivities approaching that of bulk copper.
The CVD process using aerosol precursors for the application of metal alloy thin films has been described for low temperature deposition on a variety of substrates. In Xu, C., et al., Chem. Mater. 1995, 7, 1539-1546, the CVD of Agxe2x80x94Pd, Cu-Pd, and Agxe2x80x94Cu alloys using aerosol precursor delivery over a range of preheating temperatures (70-80 xc2x0 C.) and substrate temperatures (250-300xc2x0 C.) is disclosed. There, the precursors (hfac)Ag(SEt2), (hfac)Cu1(1,5-COD), Cu(hfac)2, Pd(hfac)2 and Pd(hfac)2(SEt2), dissolved in toluene with 10% H2 in Ar as carrier gas, are used in the CVD process, a combination, which the authors claim provides advantages over traditional methods. These advantages include higher deposition rates, the ability to transport thermally sensitive compounds, and the reproducible deposition of binary materials. See also, Jain, A., et al., J. Vac. Sci. Technol. B 11(6), Nov/Dec. 1993, 2107-2113 ((CVD of copper on both SiO2 and W from (hfac)CuL, where hfac=1,1,1,5,5,5-hexafluoroacetylacetonate, and L=1,5, COD or vinyltrimethylsilane (VTMS)); and Norman, J. A. T., et al., Journal De Physique IV, Colloque C2, suppl. Au Journal de Physique II, Vol. 1, September 1991, 271-277 ((CVD of the volatile liquid complex Cu+1(hexafluoroacetylacetonate) trimethylvinylsilane, [Cu+1(hfac)TMVS])).
In an aerosol-assisted CVD process, the precursor is first dissolved in a solvent. The solution is passed through an aerosol generator, where micron-sized aerosol droplets are generated in a carrier gas and are transported into a preheating zone where both the solvent and the precursor evaporate. The precursor vapor reaches the heated substrate surface where thermally induced reactions and film deposition takes place. This method may be employed on a variety of mask-based substrates. However, distinct disadvantages of the CVD process have heretofore included a necessity to mask the substrate for deposition, the resulting large-grain-microstructures (e.g., 0.1 xcexcm to 0.6 xcexcm) of the film, and the fact that the deposition rate is precursor evaporation-rate limited in the sense that the deposition rate is related to the partial pressure of the precursor which is fixed by the vapor pressure. These limitations and others, such as the inefficient use of expensive precursor materials, inherent in the CVD process, render it incapable of forming linewidths in the range of 130 xcexcm, or less, at high deposition rates without increasing the deposition temperature.
Screen printing using metal powders and metallo-organic decomposition (MOD) compounds has also been used for metallization. For example, U.S. Pat. No. 5,882,722 describes the use of screen printable metal powders and MOD products to print thick films at low temperature. The thick films are formed of a mixture of metal powders and metallo-organic decomposition (MOD) compounds in an organic liquid vehicle in a two-step screen-print and heat process. The mixtures contain a metal flake with a ratio of the maximum dimension to the minimum dimension of between 5 and 50. The vehicle may include a colloidal metal powder with a diameter of about 10 to about 40 nanometers. The concentration of the colloidal metal in the suspension can range from about 10% to about 50% by weight. The MOD compound begins to evaporate at a temperature of approximately about 200xc2x0 C. and then consolidation of the metal constituents and bonding to the substrate is completed at temperatures less than 450xc2x0 C., in a time less than six minutes.
Direct printing, using a spray or ink jet process, however, necessitates the formulation of inks which are substantially different from those formulations which are currently used in screen printing applications. Unlike screen printable inks, the viscosity of these inks must be at or near that of water, in order to permit printing with piezoelectric or thermal ink jet systems and to prevent agglomeration of the ink on the substrate. There is also no need to have the printed line be free standing or for the ink to include binders, and the like. Volatility of these inks should also be low enough to prevent consequential solvent loss at low temperature but high enough to be readily lost when applied at the substrate temperature.
The use of metal organic precursor materials, either with or without metallic particles, for use in a process to directly write conducting metal layers or grids, using a non-vacuum deposition technique, such as spray or ink-jet application, would greatly simplify the process of applying metal films, due to a reduction in capital outlay and material costs. Ink-jet printing, for example, is also advantageous in its ability to yield very narrow grid lines with a corresponding efficient use of precursor materials, in contrast to screen printing, and a decrease in shading losses on a variety of substrates. As related to solar cells, these advantages can translate directly into cells having an increase in cell efficiency.
In direct writing, ink precursor chemistries are chosen according to the requirements of the device of interest. For example, it has been shown that Ag(neodecanoate) in xylene can be used as a precursor for ink-jet deposition of xcx9c130 xcexcm-wide Ag grids in Si solar cells. Teng, K. F. et al., IEEE Electron Device Lett. 1988, 9, 591. A post-deposition anneal in air at 350xc2x0 C. has also yielded Ag conductors with reasonable electrical properties (xcfx81xcx9c100 xcexcxcexa9xc2x7cm). The high temperature anneal step, however, is not amenable for use in other grid metallizations, such as for CuInSe2 solar cells, where temperatures in excess of 200xc2x0 C. substantially degrade the diode properties of the heterojunction owing to solid state diffusion and oxidation, or both. None of the foregoing descriptions therefore enable the use of ink precursor metal chelate compounds in a direct write process for producing fine grain electrical conductors on a substrate. Such a process would desirably reduce the number of rate-limiting steps, and allow for the low temperature deposition of finely sized metal conductors capable of narrow linewidths with adequate conductivity.
In view of the foregoing considerations, it is the object of the present invention to provide a process for the use of metal chelates in a direct write process of forming thin-film conductors which are useful in microelectronics, solar conversion technologies, and the like, and which are characterized by good interparticle structural connectivity and electrical conduction.
Another object of the invention is to provide a direct write process for spray or ink jet printing of a copper, silver, aluminum, or gold electronic contact lines on a glass or polymer substrate.
It is yet another object of the present invention to provide a direct write method for forming a metallic film having electrical conductivity and structural connectivity in CIS solar cells.
It is yet another object of the present invention to provide a Ni film having electrical conductivity and structural connectivity to a ZnO layer in CIS solar cells.
These and other objects of the present invention will become apparent throughout the description of the invention which now follows.
Briefly, the present invention provides a process for forming an electrical conductor on a substrate, consisting essentially of providing an ink comprised of a metallic chelate, printing directly thereon the ink, and decomposing the ink wherein the metal-chelate is converted to a solid metal conductor on the substrate.
Unless specifically defined otherwise, all technical or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. xe2x80x9cDirect printingxe2x80x9d and xe2x80x9cprinting directlyxe2x80x9d means printing an ink on a substrate using a pressurized jet, dip pen, spray, or nanotube printing. xe2x80x9cConformal substratexe2x80x9d means spray printing of metal coatings on flexible substrates, such as flexible circuit boards. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described.